The present invention relates generally to antennas and, more specifically, to an FM antenna that is omnidirectional, small, inexpensive and easily manufactured.
Various systems have been developed which utilize means for broadcasting signals from a central station to one or more remote stations for the purpose of communicating digital information or for controlling the operation of the remote station. Some of these systems utilize FM broadcasting technology and, more particularly, subsidiary communications authorization (SCA) channels. U.S. Pat. No. 4,199,761 which issued on Apr. 22, 1980 to Whyte et al. discloses an automated electric power distribution network that includes a radio communication system for transmitting multipurpose, mutitone signals in one or more signaling channels over the auxiliary channel or channels of one or more commercial broadcasting stations. Energy management terminals receive the signals at remote locations that are served by the electric distribution network. Radio receivers located at the terminals have frequency selective detectors that are responsive to different multitone signals. U.S. Pat. No. 3,980,954 which issued on Sept. 14, 1976 to Whyte discloses an arrangement of communication components to provide communications between a central control center and various customer load locations in an electrical power distribution system. Control or interrogation signals originate at the control center and are transmitted over a suitable facility, such as a telephone line, to an FM broadcasting station. The control signals frequency modulate an ultrasonic subcarrier which modulates the FM broadcast program material. Radio receivers at the customer load locations receive, filter and decode the broadcast signals which are used to activate the control or logic circuits associated with the customer location. U.S. Pat. Nos. 3,980,0954 and 4,199,761 are hereby incorporated by reference.
U.S. Pat. No. 4,196,310 which issued on Apr. 1, 1980 to Forman et al. discloses an FMSCA broadcasting system that includes subscriber-actuated portable receiving terminals. It describes an information broadcasting system that includes a central transmitting station for encoding data in binary form utilizing a key word and transmitting an FMSCA signal shift modulated by the encoding data. U.S. Pat. No. 4,112,369 which issued on Sept. 5, 1978 to Forman et al. discloses a similar system which comprises portable terminals that include a receiver for decoding and phase-shifting FMSCA signals into encoded binary data.
U.S. Pat. No. 4,048,619 which issued on Sept. 13, 1977 to Forman et al. discloses an information broadcasting system that includes a central broadcasting station for encoding data in binary form utilizing a key word and transmitting on one frequency an FMSCA signal phase shift modulated by the encoded data. The encoded data is also supplied to one or more remote broadcasting stations, typically via telephone lines, where it is transmitted on a second frequency as an FMSCA signal phase shift modulated by the encoded data.
U.S. Pat. No. 4,417,349 which issued on Nov. 22, 1983 to Hills et al. discloses an FMSCA data transmission system with a raised cosine filter. Digital data is transferred via the subcarrier of a commercial broadcast FM radio station. Digital data generated by an originating subscriber is shaped with substantially a raised cosine response in the time domain. The shaped signal is then pre-emphasized and used to frequency-modulate a subcarrier. The subcarrier in turn frequency-modulates the carrier and the data is transmitted by the radio station along with its commercial program.
When digital data is broadcast from a central location by an electric utility company for the purpose of performing load control functions, the electrical energy consumers are provided with a load control terminal which is configured to respond to the broadcast signals by selectively opening or closing relay contacts that control the flow of electrical power to selected appliances. Typically, the relays of a load control terminal are associated with appliances that can be shed without incurring undue hardship to the electrical energy consumer. For example, water heaters and air-conditioners are typical appliances that can be disconnected during periods of peak demand without placing undue hardship on the electrical consumer.
Radio signal receivers, such as load control terminals used by electric utility companies to regulate peak demand, require an antenna for the purpose of receiving the broadcast information. In situations where no spatial restrictions exist, the antennas can be configured to maximize their reception capabilities. However, in many applications there are severe size limitations placed on the antenna design. Load control terminals that are used in electric utility company systems which regulate customer loads present severe size limitations since the antenna is generally placed inside the load control terminal which is approximately the size of a standard electric meter. Typical load control terminals are generally cylindrical in shape and have a diameter of approximately 6 inches. An antenna that is to be placed within the load control terminal must therefore conform to these size restrictions.
Many different types of antennas are known to those skilled in the art. For example, U.S. Pat. No. 4,222,053 which issued on Sept. 9, 1980 to Newcomb discloses a vertical antenna which is omnidirectional and includes completely automatic band-switching for the amateur radio frequencies of 160 meters, 80-75 meters, 40 meters, 20 meters, 15 meters and 10 meters. The vertical antenna has a low angle of radiation and a low standing wave ratio on all frequencies which permits direct coaxial cable transmission line feed.
U.S. Pat. No. 3,335,420 which issued on Aug. 8, 1967 to Silliman, discloses a dipole antenna with combination feed-support rods. This antenna includes a support or mast section which externally has a metal tube inside of which is a rigid coaxial feed line. The coaxial feeder itself has a rigid outer section and an inner rod. Perpendicular to the mast section are two arm sections which are center-fed. Each arm section includes a hollow metal radiating or receiving cylinder closed at its outer end and open at its inner end. The metal cylinders are each much shorter than a quarter wavelength and each is connected at its inner end internally by electrical connection means including respective rod means that otherwise connect to the coaxial outer section and inner rod.
U.S. Pat. No. 3,448,454 which issued on June 3, 1969 to Lane discloses an omnidirectional circular dipole antenna that is usable to facilitate television reception on small boats. It provides an antenna which possesses nondirectional characteristics for use with television or FM receivers on a moving boat.
U.S. Pat. No. 3,155,976 which issued on Nov. 3, 1964 to Wickersham et al. discloses a broadband straight ladder antenna with twin wire balanced feed supplied via an integral unbalanced line. It is related to an improvement of the operating bandwidth of the Yagi-type antenna while preserving the compactness and structural simplicity of the well-known Yagi-Uda antenna which is one of the simplest and most compact high-gain antennas known to those skilled in the art.
U.S. Pat. No. 4,485,385 which issued on Nov. 27, 1984 to Ralston discloses a broadband antenna that features a diamond-shaped radiator. The radiator can be bent to form a selected dihedral angle to achieve omnidirectivity when a plurality of antennas having reflector screens are used.
When used in conjunction with a load control terminal, an antenna must not only be severely limited in size, but must also provide omnidirectional signal reception and be simple and inexpensive to manufacture. The present invention provides a small omnidirectional antenna that is easy to manufacture and is especially suitable for use in conjunction with load control terminals which are designed to receive and respond to digital information that is broadcast by an FM broadcasting station.
The present invention comprises a dipole segment that includes a conductive rod. This dipole segment is connected in electrical communication with a capacitive segment that comprises an inner, or central, conductor that is disposed within a cylindrical conductor, such as a braided shield. The capacitive segment of the present invention can be provided by using a preselected length of coaxial cable. The central conductor of the coaxial cable is connected in electrical communication with the dipole segment and the cylindrical conductor, or braided shield, is connected to an electrical ground. The present invention also provides an inductor that is connected in electrical communication with the central conductor of the capacitive segment being connected electrically in series between the dipole segment and the inductor. In a preferred embodiment of the present invention, the inductor is a variable inductor.
In one embodiment of the present invention, the dipole segment is a continuation of the central conductor of the capacitive segment with the cylindrical conductor, or braided shield, removed from the dipole segment portion of the present invention. The inductor of the present invention can be provided by anyone of a number of commercially available variable inductors that can be used to permit fine-tuning of the present invention by adjusting the precise inductance of the inductor after manufacture and assembly of the antenna of the present invention within a load control terminal or similar device.
The present invention is usable with a receiver wherein the inductor of the present invention is connected serially between the receiver and the central, or inner, conductor of the capacitive segment and the capacitive segment of the present invention is connected serially between the inductor and the dipole segment.